High resolution measurement of light frequency from incoherent sources typically makes use of cavity interference such as Fabry Perot interferometers or etalons, or absorption lines from some medium. Interferometers such as a Fabry Perot or an etalon are expensive and have low acceptance angles, meaning the deviation from the desired angle at which the light enters the interferometer has very little tolerance. Moreover, for such interferometers increasing the spectral resolution lowers the transmission of the signal (reduces the number of photons included in the signal). Absorption line mediums (e.g., iodine, potassium and sodium) require some atomic or molecular transition in the medium, and they only occur at discrete and fixed frequency locations. Additionally, since absorption lines absorb light, they deplete the strength of the signal being measured.
Magneto-optic spectrophotometers can be used to measure frequency, but they only distinguish light near a particular absorption line from light that is not near a particular absorption line, which provides very low frequency resolution in comparison to the current invention.